Atmospheric carbon dioxide (CO2) exists as a gas at standard temperature and pressure, and it occurs in Earth’s atmosphere in this state [1,2]. Rising levels of CO2, a greenhouse gas, are a major environmental problem [3]. The main source of CO2 emissions is the burning of fossil fuels, such as natural gas and coal. The combustion of these fossil fuels, such as natural gas, coal, and oil, is one of the major emission sources of CO2 [4]. In addition to outdoor levels of CO2, indoor levels of CO2 have been extensively investigated because they have a direct impact on human health. Certain levels of CO2 in indoor spaces cause symptoms such as headaches, vomiting, drowsiness, and dyspnea. Certain indoor spaces, such as automobiles, subways, and aircraft, have high concentrations of CO2, which can cause serious health hazards. Thus, an increased CO2 concentration in these types of spaces is considered by many researchers to be a serious problem [5,6]. Recently, CO2 capture and storage technology, including adsorption, absorption, cryogenics, and membranes, has received increased attention motivated by environmental concerns [7-9]. Absorption into an alkanolamine solution is a conventional and mature technology; however, this method has major drawbacks, such as high regeneration energy requirements and significant waste. However, adsorption using solid sorbents has attracted widespread interest over recent years because of its low energy consumption, lower regeneration requirements, and ease of conservation and recycling [10]. A wide range of solid adsorbents are currently under investigation, including zeolites, metal organic frameworks, porous carbon materials, macro-porous silica, and certain metal oxides [11-15]. Among the solid adsorbents for CO2 capture, carbon materials, such as activated carbon (AC), carbon aerogels, graphene, and carbon nanotubes, have shown significant advantages because of their highly developed porosities and extended specific surface areas [16-24]. AC is of considerable interest because it is inexpensive, readily available, easily tuned, and thermally stable [25-27]. Surface modification can be performed via a range of treatments, such as chemical or electrochemical oxidation, chemical activation, and thermal treatment [28-30]. Chemical activation by ZnCl2, NaOH, or KOH is an effective method of controlling the poresize distributions and specific surface areas of carbon supports [31-35]. In particular, the chemical activation method using KOH has been the most widely studied. Researchers have reported that high specific surface areas and well-developed microporous structures can be obtained. Also, KOH activation is the most effective method in terms of the degree of activation. In this study, we investigated the effects of KOH activation at various ratios on the surface of AC. Samples were prepared to evaluate highly efficient CO2 capture capacities. The KOH impregnation process was initiated by mixing 1 g of AC with 4 g of KOH in a water and ethanol solution (1:1). The resulting solution was stirred to ensure even mixing and vaporized for 48 h at 90℃. Then, the products were placed in a drying oven at 80℃ overnight. The activation process was performed in a tube furnace under N2 flow (200 mL/min); it involved heating the sample at a rate of 3 ℃/min up to 900℃ and maintaining this temperature for 1 h. The activated product was washed with a 2 M HCl solution and distilled water, filtered until it reached a pH of 7, and dried for 24 h at 80℃. The activated AC (i.e., KOHAC) samples prepared at AC:KOH ratios of 0, 1:1, 1:2, 1:3, and 1:4 are here referred to as pristine-AC, K1-AC, K2-AC, K3-AC, and K4-AC, respectively. The porous textures of the samples were determined from the N2 adsorption-desorption isotherms at 77 K using a specific surface area and pore size analyzer (BELSORP, Osaka, Japan). Before the measurements, the prepared samples were outgassed at 200℃ for 12 h under high vacuum through the degas port of the adsorption instruments. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area, and the total pore volume (Vt) was obtained from the N2 adsorption volume at P/P0 = 0.99. The samples were evaluated for their CO2 capture capacities using the BELSORP instrument. The samples were degassed at 200℃ for 12 h and cooled to the required adsorption temperature of 25℃. Then, the CO2 adsorption volume was measured at a specific relative pressure (P/P0) at 298 K. The indoor CO2 capture capacities of the samples were determined in a manufactured chamber at the laboratory scale. The initial concentration of CO2 was about 400 ppm. The gas flow rate of CO2 was adjusted using a flow controller. The atmosphere in the chamber was maintained with circulating CO2 at a fixed flow rate of 8 mL/s for 10 min. During this process, the concentration of CO2 was recorded every minute. The chamber test was conducted using a CO2 analyzer (IAQ8494; Tech one, Seoul, Korea). N2 adsorption/desorption isotherms and meso- and micropore size distribution curves were used to assess the textural surface properties of the prepared samples. From Fig. 1, it is evident that the prepared samples had Type I adsorption isotherms (IUPAC classification). Also, most of the pores had relative pressures below 0.1 as well as markedly increased adsorption amounts, indicating that these samples mainly consisted of well-developed micropores. As shown in Table 1, all prepared samples possessed BET surface areas of 800-1500 m2/g. The total pore volumes and mesopore volumes tended to increase with increasing amounts of KOH up to the quantity in the K2-AC sample. The K2-AC sample exhibited a large specific surface area (1507 m2/g) and high total pore volume (0.670 cm3/g) and micropore volume (0.580 cm3/g). However, the surface areas, total pore volumes, and micropore volumes of the K3-AC and K4-AC samples were lower, likely because they contained excess KOH; large amounts of KOH destroyed the pore structures and increased the mesopore volumes. Fig. 1. N2 full isotherms at 77 K porosity parameters of the Pristine-AC and K-AC samples Table 1. Textural properties of Pristine-AC and K-AC samples a SBET : specific surface area computed using BET equation (P/P0 = 0.05-0.1). b Vtotal : total pore volume (P/P0 = 0.990). C Vmeso : micropore volume determined from the BJH equation. d Vmicro : mesopore volume determined from the subtraction of mesopore volume from total pore volume. e Fmicro : (micropore volume/total pore volume) × 100 As shown in Fig. 2a, the mesopore volume increased with increasing amounts of KOH. All samples had very sharp peaks at approximately 2 nm because they consisted mainly of micropores. As the amount of KOH increased, the mesopore volume rapidly increased. Fig. 2b shows the micropore size distribution of the pristine-AC and K-AC samples. Thus, micropores developed around pores with diameters between 0.6 and 0.8 nm, proving that they primarily developed in the parts of the prepared K-AC samples with pore diameters <1 nm. Fig. 2. Meso-(a) and micro-(b) pore size distribution of the Pristine-AC and K-AC samples Fig. 3 shows the CO2 capture capacities of the pristine-AC and K-AC samples at 298 K and 1 bar. The CO2 capture capacity of pristine-AC was 118.75 mg/g, while the K2-AC sample had the highest CO2 capture capacity of 144.5 mg/g at 298 K and 1 atm. The CO2 capture capacities of the samples decreased in the following order: K2-AC > K1-AC > K3-AC > K4-AC > pristine-AC, as shown in Fig. 3. The improvement of the CO2 capture capacities resulted from the development of micropores and increased specific surface areas. However, in the cases of K3-AC and K4-AC, the CO2 capture capacities were reduced because of the destruction of the pore structures by excess KOH. Therefore, the amount of KOH significantly affected the CO2 capture capacity. Chemical activation using KOH increased the CO2 capture capacity by controlling the meso- and micro-pore volumes and specific surface areas. Fig. 3. CO2 adsorption isotherms of the Pristine-AC and K-AC samples at 1 bar and 298 K. Fig. 4 shows the indoor CO2 capture capacities of the pristine-AC and K-AC samples, as determined using the chamber test. The chamber test was conducted in a manufactured chamber at the laboratory scale. Fig. 4a shows that the initial CO2 concentration of each sample test was about 400 ppm. The atmosphere in the chamber was maintained with circulating CO2 at a fixed flow rate of 8 mL/s for 10 min using a flow controller. After 10 min, the CO2 concentration of the non-adsorbents, i.e., pristine-AC, and the prepared samples, i.e., K1-AC, K2-AC, K3-AC, and K4-AC, were 4060, 3459, 3020, 2756, 3140, and 3445 ppm, respectively. As expected, the adsorbents made a significant difference, with a maximum concentration reduction of 1304 ppm. Fig. 4. Indoor CO2 capture capacity of the Pristine-AC and K-AC samples. Fig. 4b presents the same results as Fig. 4a; however, Fig. 4b shows them as percentages of the indoor CO2 capture capacity. The indoor CO2 capture capacity rates decreased in the following order: K2-AC > K1-AC > K3-AC > K4-AC > pristine-AC. Therefore, the amount of KOH affected the indoor CO2 capture capacity rate. K2-AC featured the highest indoor CO2 capture capacity rate of approximately 30%, because of its well-developed micropore size (approximately 0.68 nm) with an appropriate amount of KOH. It can be concluded that pore size of 0.68 nm may the optimal value for indoor CO2 capture. In contrast, K4-AC had a significantly lower indoor CO2 capture capacity rate of 16%, which was the same as that of pristine-AC. An appropriate amount of KOH improved the indoor CO2 capture capacity rate; however, excess KOH resulted in the destruction of the pore structures. We investigated the effects of activation using various amounts of KOH on the surface of AC. The K2-AC sample exhibited a large specific surface area (1507 m2/g) and high total pore volume (0.670 cm3/g) and micropore volume (0.580 cm3/g). The micropores in the K-AC samples appeared primarily in parts where the pore diameters were < 1 nm. One of the prepared K-AC samples, namely, K2-AC, exhibited a high CO2 capture capacity of 144.5 mg/g and an indoor CO2 capture capacity rate of about 30%. The excellent CO2 capture capacity and high indoor CO2 capture capacity rate were attributed to the optimized amount of KOH.
